1. Field of the Invention
The present invention relates to an image forming apparatus for correcting a position deviation of an image to be formed.
2. Description of the Related Art
A position deviation (color deviation) is known as a problem in a color image forming apparatus of a tandem system. The tandem system is a system which is provided with developing machines and photosensitive elements each number of which is the same as the color number of the toner and sequentially transfers images in different colors onto a carrier belt or a print medium. The position deviation means the event that the image is made to be transferred in a position different from a position to be expected. A cause of the position deviation is unevenness or a mounting position deviation of a lens in a deflection scan apparatus, or an assembling position deviation of the deflection scan apparatus to a color image forming apparatus body. Caused by this position deviation, an inclination or a curve is generated in a scan line and a degree of the inclination or curve differs in each color, thus creating the position deviation (color deviation) of the image.
An example of the method for overcoming this problem includes a method in which magnitudes of an inclination and a curve in a scan line are measured by using an optical sensor and the bit map image data is corrected to cancel out the magnitudes, forming the corrected image (Japanese Patent Laid-Open No. 2004-170755).
In general, a color image forming apparatus forms a pattern for position deviation detection for each color on an image carrier such as a photosensitive element or an intermediate transfer element or on a transfer material carrier such as a carrier belt and detects the formed pattern by optical sensors located at both sides in the downstream portion. Further, correction of the position deviation is made based upon the detection result.
However, in a case where the pattern for position deviation detection is formed on the transfer material carrier by the position deviation correcting method using the process described in Japanese Patent Laid-Open No. 2004-170755, in some cases the detection accuracy of the pattern is deteriorated. Hereinafter, the reason for the deterioration of the detection accuracy will be explained.
In a case of finding a position deviation (color deviation) using a pattern for position deviation detection, a pattern configured by horizontal lines, inclined lines or the like is formed, a position of the line in the pattern is detected by a scan of an optical sensor on the line, and the position deviation is calculated based upon the detected position of the line. FIG. 11A shows an example of a pattern for position deviation detection to be formed on a transfer material carrier. K1 and K2 are patterns for position deviation detection formed in black, and Y is a pattern for position deviation detection formed in yellow. Respective detection timings of lines in the patterns K1, K2, and Y by the optical sensor are defined as t1, t2, and t3 and a conveying speed of an intermediate transfer element is defined as v. Then the position deviation in a sub scan direction between the black and yellow patterns can be found according to the formula of {t3−(t1+t2)/2}×v. The position deviation in the sub scan direction means a deviation in an arrow direction (scan line direction of the optical sensor) of FIG. 11A. The patterns in this example are configured only by the black and yellow patterns, but a position deviation in a sub scan direction between similar patterns in other colors can be also found.
The detection timing of each line is found by a signal from the optical sensor. FIG. 11B shows horizontal lines formed as a part of the pattern for position deviation detection and a scan line of the optical sensor, and FIG. 11C expresses a signal from the optical sensor in a case of detecting the horizontal line in FIG. 11B along the scan line. As shown in FIG. 11C, timings where the signal and a threshold value (for example, threshold value of the density) intersect are defined as detection timings of edges in the line and the detection timings at both ends of the line are averaged, thus finding a detection timing of a center position of the line in a sub scan direction. The detection timing in the center position is defined as the detection timing of the line.
Here, in a case of detecting the pattern for position deviation detection to which the position deviation correction using the process described in Japanese Patent Laid-Open No. 2004-170755 is made, based upon the method as described above, there possibly occurs a problem that the detection accuracy is deteriorated. This problem will be explained with reference to FIGS. 12A and 12B.
FIG. 12A shows an image forming state in a case where the position deviation correction is made to the horizontal line, and FIG. 12B shows a detection signal in a case of detecting the line in FIG. 12A by the optical sensor. As shown in FIG. 12A, in a case where the position deviation correction is made, a pixel smaller than one pixel is used at an edge portion of the line in addition to the correction in a pixel unit. Details of the correction method are described in Japanese Patent Laid-Open No. 2004-170755.
Here, since it is generally difficult to stably form a small pixel due to the influence of the fluidity in a toner in an image forming apparatus of an electronic photography system, the edge portion of the line configured by the small pixel tends to be easily unstable.
In a case of forming the small pixel, when an optical amount at exposure is made small or exposure hours are shortened, a latent image is made shallow and therefore an applied amount of the toner is unstable. Therefore, as compared to an ideal detection signal shown in a dotted line in FIG. 12B, the edges of the line are detected to be deviated in an actual detection signal shown in a solid line. As a result, since the aforementioned center position of the line is also calculated to be deviated, the detection accuracy of the pattern for position deviation detection is deteriorated.
In addition, not limited to the aforementioned correction, for example, even in a case of making a correction in a pixel unit alone, caused by the correction a defect possibly occurs in the position deviation detection result. For example, that is a case where position deviation detection patterns to be formed overlap. In this way, in a case where a digital position deviation correction of correcting the image data itself is made in a sub scan direction, the defect occurs in the detection result of the pattern for position deviation detection.